You can refer to these steps below.
1. Please go to Main Menu-->System-->General to change video standard to have a try. If it doesn't work, change it back to the previous one.
2. Please connect only one camera to the recorder, because maybe power has some problem and can't support all cameras working or there is signal interference between cameras.
3. Please change another monitor or TV to check if it is the monitor's or TV's problem, not a recorder or camera.
4. Please check the review on the phone or computer face the same problem.
You can refer to this link below to view on a computer.
You can refer to this link below to view on phone.
5. Please change to another power supply to have a try.